1. Field of the Invention
The present invention relates to an optical modulator, an optical transmission device, and a bias adjusting method.
2. Description of the Related Art
A conventional optical modulator that performs a Quadrature Phase Shift Keying (QPSK) modulation includes two sub-modulators (see, for example, Japanese Patent Application Laid-open No. 2008-187223 and Japanese Patent Application Laid-open No. 2008-236512). Signals of an I channel and a Q channel are input to these two sub-modulators, respectively. The two sub-modulators modulate the input signals of the I channel and the Q channel and output optical modulation signals. Among the optical modulation signals of two systems modulated by the two sub-modulators, a phase of one optical modulation signal is rotated by π/2 (90 degrees) by using an optical phase modulator. The optical modulation signal of which a phase is rotated is combined and interfered with an optical modulation signal modulated by the other sub-modulator, thereby generating a QPSK signal.
In such an optical modulator, it is important to rotate a phase by π/2 by using the optical phase modulator. Therefore, to ascertain whether a phase is correctly rotated by the optical phase adjuster, a monitor photo diode (PD) is provided in the conventional optical modulator to detect an optical modulation signal after a synthesized output by applying a signal of a low frequency pulse to a bias voltage to be applied to the sub-modulator. The optical modulator ascertains a phase state of the optical modulation signal by using the optical modulation signal detected by the monitor PD. When the optical phase adjuster is set to correctly change a phase by “π/2”, the I channel and the Q channel are orthogonal to each other, and phases are offset each other and there occurs no variation in optical output power even when a phase is modulated by being applied with a low-frequency pulse signal. On the other hand, when the phase change is deviated from “π/2”, a variation occurs in a level of an optical modulation signal along with a low-frequency pulse signal. A phase rotated by the optical phase adjuster is adjusted by using this operation, and a correct phase rotation can be achieved by finding an optimally adjusted amount (an adjusted amount to set a phase difference between an optical modulation signal in the I channel and an optical modulation signal in the Q channel to “π/2”) and by setting the optical phase adjuster at the optimally adjusted amount.
While the sub-modulator described above is constituted in many cases by lithium niobate (LiNbO3, hereinafter, “LN”), the development of constituting a sub-modulator for an integrated modulation device using compound semiconductors has been also progressed. This integrated modulation device uses a compound semiconductor such as indium phosphide (InP) for a substrate. Optical elements such as an optical waveguide, a phase modulator, an amplifier, an optical multiplexer/demultiplexer, a laser can be formed as one unit by stacking active layers on a compound semiconductor substrate and etching unnecessary parts. A compound semiconductor substrate can be used to manufacture various elements as compared with elements made of LN, and also has an advantage of being compact as a whole. Therefore, a compact and integrated optical modulator can be formed by forming various elements in addition to a sub-modulator on a compound semiconductor substrate and by connecting these elements by an optical waveguide.
However, when an optical modulator is integrated and unified on a compound semiconductor, a loss in a sub-modulator is larger than that when a sub-modulator is formed by LN, and a variation of the loss becomes large. Therefore, a loss in an optical modulator increases, and a variation of a loss between sub-modulators becomes larger.
It is desired that an amplitude level of an optical signal in the I channel and that of an optical signal in the Q channel are equivalent. However, when there is a variation between a loss in a sub-modulator for the I channel and that in a sub-modulator for the Q channel, a difference occurs between an amplitude level of an optical modulation signal in the I channel and that of an optical modulation signal in the Q channel, and the modulation waveform quality becomes low.